The study of cancer risk is a major component of environmental epidemiology. The Epidemiology Branch emphasizes the development of methods for assessing environmental exposures, and uses biochemical measures of exposure or disease markers to evaluate cancer risk. A study of the relationship of childhood and/or adulthood passive exposure to cigarette smoke and cancer risk has been completed, and data collected for this study are now being used to evaluate the quality of exposure histories obtained from surrogate respondents. Additional studies of "passive smoking" have been started. These include a prospective study of mortality and cancer incidence among persons who were living with smokers in 1963, and measurements of indicators of mutagenesis and genotoxicity in nonsmokers, passive smokers, and smokers. A study of risk factors for acute leukemias in adults has been started in collaboration with Cancer and Leukemia Group B (CALGB), a collaborative cancer treatment group with more than 20 member institutions. This study has an emphasis on identifying risk factors for cytogenetically defined subgroups of acute leukemia and will utilize the results of bone marrow cytogenetics being done as part of CALGB treatment protocols to classify patients into potentially etiologically distinct subgroups. The study is intended to confirm previously reported associations between occupational and environmental exposure to chemicals and risk for specific types of leukemia marked by clonal chromosomal changes. The study will also evaluate leukemia risk from childhood exposure to cigarette smoke. This association was demonstrated in the program's recently completed study of cancer risk from passive smoking. Cigarette smoke exposure, too, may be more strongly associated with specific leukemia subgroups that are defined by cytogenetic abnormalities.